The switching power supply, and the control IC for controlling the switching power supply, are provided with a current limiting function, an overcurrent protection function, an overload protection function, and such protection functions prevent components thereof, such as a power MOSFET, a diode and an inductor from breakdown.
FIG. 4 is a circuit diagram showing the circuit configuration of a power supply of an insulation type that employs a transformer for electric power conversion. FIG. 5 is a block circuit diagram of a conventional control IC for controlling a switching power supply. AC power supply input terminals IN1 and IN2 are shown in FIG. 4. Diode bridge DB is connected to input terminals IN1 and IN2 via fuse F. The rectified DC electric power is fed to the first end of primary winding Lp of transformer T1 from the DC output terminal of diode bridge DB. The second end of primary winding Lp is connected to the drain of switching device Q1 that is a power MOSFET and such a power semiconductor device. The source of switching device Q1 is grounded via sensing resistor Rs. Smoothing capacitor C1 is connected to the DC output terminal of diode bridge DB in parallel with a series circuit consisting of primary winding Lp, switching device Q1 and sensing resistor Rs.
Auxiliary winding Lb is disposed on the side of primary winding Lp in transformer T1. Rectifying diode D1 and smoothing capacitor C2 are connected to auxiliary winding Lb. The connection point of rectifying diode D1 and smoothing capacitor C2 is connected to the DC output terminal of diode bridge DB via resistor 11. Power supply Vcc is fed to the control IC via the connection point.
Generally, the control IC is provided with a plurality of external terminals (pins). The control IC in FIG. 4 includes input terminal FB as a first pin, to which a feedback voltage VFB is inputted, sensing input terminal IS as a second pin, to which a voltage signal Vis is fed, ground terminal GND as a third pin that feeds a reference voltage to each section in the control IC, output terminal OUT as a fourth pin, from which a control signal is outputted to switching device Q1, and power supply terminal VCC as a fifth pin, from which electric power is fed to the control IC. Where the control IC package has eight pins, the pins which exhibit terminal functions other than those described above include a pin employed as an input terminal for detecting zero current, a pin employed as a terminal for feeding a starting current, and a pin employed as an unconnected terminal.
The collector terminal of phototransistor PT1, which constitutes a photocoupler, is connected to input terminal FB, to which feedback voltage VFB is fed. The emitter terminal of phototransistor PT1 is grounded. Sensing input terminal IS, to which voltage signal Vis is fed, is connected to the first end of resistor R12 and the first end of capacitor C3. The second end of resistor R12 is connected to the connection point of switching device Q1 and sensing resistor Rs. The second end of capacitor C3 is grounded. Voltage signal Vis, which is proportional to drain-source current Ids flowing through switching device Q1, is inputted to sensing input terminal IS of the control IC. A PWM signal is outputted from output terminal OUT to switching device Q1 via resistor 13.
DC output terminal Vout is disposed on the secondary side of transformer T1, with a rectifying circuit interposed between secondary winding Ls and DC output terminal Vout. The rectifying circuit includes diode Ds and smoothing capacitor C4 and rectifies the voltage generated across secondary winding Ls. The anode of diode Ds is connected to the first end of secondary winding Ls. The cathode of diode Ds is connected to DC output terminal Vout and to the first end of smoothing capacitor C4. The second end of smoothing capacitor C4 is grounded. The second end of secondary winding Ls is connected to ground terminal Gnd. A load circuit (not-shown) is connected between DC output terminal Vout and ground terminal Gnd. A series circuit, consisting of three resistors R14, R15 and R16, and an output detection circuit, consisting of resistor R17, photodiode PD1 and shunt regulator D2, are connected between DC output terminal Vout and ground terminal Gnd. Photodiode PD1 constitutes the photocoupler.
The control IC that controls the power supply described above will now be described below with reference to FIG. 5. Only the first pin employed as input terminal FB, the second pin employed as sensing input terminal IS and the fourth pin employed as output terminal VOUT are shown in FIG. 5.
Input terminal FB is grounded via a resistance circuit consisting of resistors R1 and R2 connected in series with each other. The connection point of resistors R1 and R2 is connected to the first (−) input terminal of current comparator 1, which also exhibits an overcurrent limiting function. Current comparator 1 constitutes a comparator circuit having three input terminals. The second (−) input terminal of current comparator 1 is connected to reference voltage supply Vth1. The third (+) input terminal of current comparator 1 is connected to sensing input terminal IS, from which voltage signal Vis is fed. The output terminal of current comparator 1 is connected to reset terminal R of flip-flop circuit 2, and the output terminal of oscillator circuit (OSC) 3 is connected to set terminal S of flip-flop circuit 2. The output from terminal Q of flip-flop circuit 2 is subjected to impedance conversion by buffer circuit 4 and outputted from output terminal VOUT as a switching signal for switching device Q1, e.g. a power MOSFET connected externally to the control IC.
The control IC drives the gate of switching device Q1 by switching the potential of output terminal VOUT between the high (H) level and the low (L) level, such that the power supply generates a smoothed DC voltage, smoothed on the secondary side of transformer T1, between output terminal Vout and ground terminal Gnd. As a drain current flows through switching device Q1 during the ON-period thereof, a current flows through primary winding Lp on the primary side of transformer T1, connected to switching device Q1, and the current value increases, storing energy in transformer T1. Although switching device Q1 is turned off subsequently, the energy stored in transformer T1 causes a current to flow to smoothing capacitor C4 through diode Ds on the secondary side of transformer T1 during the OFF-period of switching device Q1. Thus, a smoothed DC voltage, smoothed on the secondary side of transformer T1, is generated between power-supply output-terminal Vout and ground terminal Gnd. The drain-source current Ids of switching device Q1 on the primary side of transformer T1 is converted to a voltage by sensing resistor Rs. The converted voltage is inputted as voltage signal Vis to sensing input terminal IS of the control IC. In current comparator 1, to which voltage signal Vis is inputted, a first comparison is conducted between the voltage value of reference voltage supply Vth1 and the divided voltage signal obtained by dividing the feedback voltage VFB with resistors R1 and R2 as shown in FIG. 5. In addition, a second comparison is conducted between voltage signal Vis and the lower one of the voltage value of reference voltage supply Vth1 and the divided voltage signal. The result of the second comparison is outputted to flip-flop 2.
When voltage signal Vis is compared with the divided voltage signal of feedback voltage VFB, the comparison is conducted to control the output voltage from the power supply to be at a constant value. When voltage signal Vis is compared with the voltage value of reference voltage Vth1, the conversion is conducted to detect a peak current.
A voltage signal is generated across the series resistance circuit consisting of resistors R14, R15 and R16. This voltage signal is generated corresponding to the current flowing through the load (not-shown) connected between DC output terminal Vout and ground terminal Gnd (in other words, the voltage signal is generated according to the load weight). A current corresponding to the voltage signal therefore flows through photodiode PD1 in the output detection circuit. Hence, it is possible to feed feedback voltage VFB to phototransistor PT1 connected to input terminal FB of the control IC. Feedback voltage VFB corresponds to the load current variation caused in the load circuit. In detail, as the current flowing through the load circuit increases, the electric charge discharged from smoothing capacitor C4 increases. As the output voltage of the power supply (the voltage across capacitor C4) is reduced, feedback voltage VFB rises. In contrast, as the current flowing through the load circuit decreases, the electric charge discharged from smoothing capacitor C4 decreases. As the output voltage from the power supply (the voltage across capacitor C4) rises, feedback voltage VFB is reduced.
The control IC compares voltage signal Vis inputted thereto with the voltage value of reference voltage supply Vth1. The control IC stops the switching operation when a current higher than the threshold value flows through switching device Q1 on the primary side of transformer T1. Thus, the control IC prevents an overcurrent from flowing through the component parts of the power supply in every switching cycle. The control method described above is called “current limitation control of pulse-by-pulse type”.
The control IC exhibits a latching function for latching the PWM signal output to switching device Q1 to stop switching device Q1 when the current limitation function described above continues for a certain period. The control IC also exhibits an automatic restart function to automatically resume switching operation of the switching device Q1 after the latch is removed.
When the load circuit connected to the switching power supply as described above is an inductive load, through which a high current flows only at the start, a large difference arises between the average load current and the peak load current. To protect the switching operation in such a case as described above, it is necessary to set two reference voltages, based on which the average load current and the peak current are monitored independently.
As the average load current is detected outside the conventional control IC, the detection circuit for detecting the average load current is not described herein. The result of comparing the average load current with the protection operation reference is often synthesized with the feedback signal for the PWM control. The feedback signal for the PWM control is prepared from the output voltage on the secondary side of the power supply. Then, the synthesized signal is inputted to input terminal FB of the control IC.
The current flowing through switching device Q1 is proportional to the output current on the secondary side. Therefore, the overcurrent protection apparatus disclosed in the Unexamined Japanese Patent Application Publication No. Hei.11 (1999)-215690 (Paragraphs [0015] through [0026], FIG. 4) detects an overcurrent by employing current detection circuit 11 on the primary side when the current value detected by current detection resistor RSP exceeds maximum current value I2 on the secondary side. When current detection circuit 11 detects an overcurrent, the overcurrent protection apparatus disclosed therein limits the output current by the PWM control. Current detection circuit 12 on the secondary side compares the current value detected by resistor RSS with current value I1 defined as an average current quantity. When the detected current value exceeds current value I1, pulse-width detection circuit 13 operates. As the output from pulse-width detection circuit 13 is inverted, the overcurrent protection apparatus disclosed in the Unexamined Japanese Patent Application Publication No. Hei.11 (1999)-215690 judges that the detection signal from current detection circuit 12 is inputted continuously for a period longer than the predetermined time width and limits the output current by the PWM control. As described above, the overcurrent protection apparatus disclosed therein directly monitors the load current with current detection circuit 12 on the secondary side. The overcurrent protection apparatus feeds back the averaged signal to the control section of switching device Q1 to conduct the protection operation. The overcurrent protection apparatus disclosed therein also monitors the overcurrent caused through switching device Q1 with current detection circuit 11 on the primary side. The overcurrent protection apparatus feeds back the monitored overcurrent to the control section of switching device Q1 to conduct the protection operation (in this paragraph, the reference numerals and symbols described in FIG. 4 of the Unexamined Japanese Patent Application Publication No. Hei.11 (1999)-215690 are employed).
Unexamined Japanese Patent Application Publication No. Hei.11 (1999)-234892 (Paragraphs [0022] through [0044], FIGS. 4 through 1) discloses an overcurrent protection apparatus that includes an overcurrent detection circuit for measuring the output current on the secondary side. The overcurrent protection apparatus disclosed therein starts a controlled protection operation when the detection signal from the overcurrent detection circuit exceeds the threshold value for a certain period.
As the component parts for monitoring the load current on the secondary side are additional in the conventional power supply that includes the detection circuits disclosed in the Unexamined Japanese Patent Application Publication No. Hei.11 (1999)-215690 or Unexamined Japanese Patent Application Publication No. Hei.11 (1999)-234892, it is difficult to realize the conventional power supplies disclosed in these references as inexpensive products. As the volume of the power supplies disclosed in the these references is increased relative to conventional power supplies, it is not preferable to incorporate such power supplies into small electronic instruments.
In view of the foregoing, it would be desirable to provide a control IC for controlling a switching power supply that facilitates reducing the number of component parts, the volume of the power supply, and the manufacturing costs of the power supply. It would be also desirable to provide a switching power supply that facilitates reducing the number of component parts, the volume of the power supply, and the manufacturing costs thereof.